Inflatable subreflector support for cassegrainian antenna



Nov. 15, 1966 M. LUTCHANSKY 3,235,267

INFLATABLE SUBREFLECTOR SUPPORT FOR CASSEGRAINIAN ANTENNA Filed Jun 17, 1964 r 2 Sheets-Sheet 1 lNl/EN TOP M. L U TC HANSK V A 7'TORNEV Nov. 15, 1966 M. LUTCHANSKY 3,286,267

INFLATABLE SUBREFLECTOR SUPPORT FOR CASSEGRAINIAN ANTENNA Filed June 17, 1964 2 Sheets-Sheet 2 FIG. .3

FIG. 2

United States Patent 3,286,267 INFLATABLE SUBREFLECTOR SUPPORT FOR CASSEGRAINIAN ANTENNA Milton Lutchansky, Morristown, N.J., assignor to Bell Telephone Laboratories, Incorporated, New York,

N.Y., a corporation of New York Filed June 17, 1964, Ser. No. 375,886 6 Claims. (Cl. 343-781) This invention relates to antennas and in particular to Cassegrainian antennas.

The Cassegrainian microwave antenna, which includes a main reflector and a subreflector, is based on the principles of the Cassegrainian optical telescope. Such antennas have a number of characteristics that make their use desirable for receiving relatively weak signals. Good reception, however, requires that their subreflectors maintain substantially constant angular positions with respect to their main reflectors for all elevational angles in order to maintain pointing accuracy. This requirement is met by providing sturdy supporting structures for the subreflectors.

Although rigid subreflector supporting structures overcome the above-mentioned problem, such structures frequently present other problems. Firstly, since subreflector supporting structures are within the antenna radiation patterns, radio frequency (RF) energy is absorbed and reradiated as noise. Secondly, such structures reflect R.F. energy into other noise sources which reradiate some of the energy as noise. Furthermore, since they are in the antenna radiation patterns, they cause a scattering of RF. energy which reduces the effective gains of the antennas.

Attempts to minimize the above-described effects are found in the prior art. Materials having relatively low R.F. energy absorption and reflective qualities have been used. Furthermore, structural designs have been produced to reduce the amount of structural material within the radiation patterns of the antennas. Although these attempts have produced satisfactory results in some applications, the noise and reflection problems still are of significant importance in other applications. This is especially true in the larger antennas designed for the reception of extremely weak signals.

An object of the present invention is to further reduce the noise and reflection problems introduced by Cassegrainian antenna subreflector supports.

This and other objects are accomplished by both further reducing the amount of structural material in the radiation patterns and making a greater portion of the subreflector supports out of material which is substantially transparent to RP. energy.

In one of its broader aspects, the invention takes the form of a Cassegrainian antenna in which an air inflated membrane holds the antenna subreflector in front of the main reflector. This membrane is substantially transparent to RF. energy. A pair of guy wires are connected between the main reflector and circumferential locations on the subreflector with the guy wires lying in a vertical plane passing through the midpoints of the reflectors. The membrane and guy wires remain under tension for all elevational angles of the antenna. Furthermore, these tensions maintain the subreflector substantially axially concentric with the main reflector when the antenna is in a vertical elevational position.

When the antenna elevational angle is other than vertical, the inflated membrane is in effect performing as a cantilevered beam and tends to arc as a result of the transverse load applied to it by the subreflector. The combination of the inflated membrane with the guy wires causes a portion of the weight of the subreflector to be assumed by the wires, with one of the wires under greater tension than the other wire. The difference in tensions in the wires causes a moment to be applied to the subreflector. This moment prevents the subreflector from rotating with respect to the main reflector when the inflated membrane arcs.

In accordance with the invention, the moment produced by the wires is used to advantage by using materials having particular stiffness characteristics for the inflated membrane and wires. These stiffness characteristics cause the loading transverse to the axis of the membrane to be shared between the membrane and the wires in such a way that the moment applied to the subreflector by the wires for an elevational position of the antenna other than vertical, maintains the angular position of the subreflector with respect to the main reflector the same as when the antenna is in a vertical position. When using materials having such stiffness characteristics, the transverse loading is shared so that the moment applied by the wires to the subreflector is just sufficient for all elevational angles of the antenna to maintain the subreflector in the desired angular position with respect to the main reflector.

As recognized by those skilled in the art, thin structural members under tension provide greater structural strength than the same size members under compression. Because, therefore, the invention comprises a unique structural configuration which uses thin members under tension, use of the invention results in a reduction of structural material in subreflector supports with a resulting improvement in RF. energy absorption and reflection characteristics.

Other objects and features of the invention will become apparent from a study of the following detailed descriptions of several specific embodiments.

On the drawings:

FIG. 1 discloses one embodiment of the invention;

FIGS. 2 and 3 are cross sectional views of a portion of the embodiment of FIG. 1 when that embodiment is in vertical and horizontal positions, respectively.

The embodiment of FIG. 1 includes a two-tier base 10 on which an azimuth turntable 11 is mounted. A plurality of bearing rollers 12 between the turntable and base permit the turntable to be moved in azimuth with respect to the base. A pedestal 13 is aflixed on turntable 11. Pedestal 13 includes a horizontal shaft 14 on which a back up structure 15 for a parabolic reflector 16 is mounted.

A subreflector assembly 17 is supported in front of reflector 16 to form a Cassegrainian antenna. The supporting structure for the subreflector assembly comprises a truncated cone-shaped inflated membrane 18 and a pair of guy wires 19 and 20. Guy wires 19 and 20 are connected between parabolic reflector 16 and subreflector assembly 17 so that the wires lie in a vertical plane passing through the midpoints of reflector 16 and subreflector assembly 17. Subreflector assembly 17 is affixed in a substantially airtight manner in the smaller end of truncated cone-shaped membrane 18 while the larger end is affixed in a substantially airtight manner to reflector 16. The source of compressed air for inflated membrane 18 is contained in pedestal 13. Pedestal 13 also includes receiver and transmitter equipment and a source of mechanical energy for directing the Cassegrainian antenna in azimuth and elevation.

The manner in which the supporting structure for subreflector assembly 17 functions may be better understood by referring to FIGS. 2 and 3 which show sectional views of the antenna in vertical and horizontal positions, respectively. As shown in these drawings, subreflector assembly 17 comprises a subreflector 21 set in a mounting ring 22. The outer periphery of ring 22 is secured in a substantially airtight manner in the narrow end of the truncated cone-shaped inflated membrane 18 with subreflector 21 directed toward parabolic reflector 16. The larger 6 end of inflated membrane 18 is aflixed in a substantially airtight manner to reflector 16.

Reflector 16 has an opening in its center through which energy received and to be radiated is passed in accordance with the principles of operation of a Cassegrainian antenna. A feed line 23 is mounted on the convex side of and over the opening in reflector 16. The other extremity of feedline 23 is connected to receiving and transmitting equipment contained in pedestal 13 of FIG. 1. A flexible hose 24 passes through reflector 16 to supply compressed air to inflate membrane 18. The other extremity of hose 24 is terminated in a source of compressed air in pedestal 13 of FIG. 1.

Guy wires 19 and 20 and inflated membrane 18 remain under tension for all elevational angles of the antenna. Furthermore, when the antenna is in a vertical position as shown in FIG. 2, these tensions cause subrefiector 21 to be in substantial axial alignment with reflector 16.

When the antenna is in a position other than vertical, the inflated membrane, in effect, performs as a cantilever beam. In other words, the inflated membrane arcs or sags as a result of the load applied to it primarily by subrefiector assembly 17. This is illustrated in FIG. 3 where the center of subrefiector 21 has fallen below the axial centerline of reflector 16.

In the absence of guy wires 19 and 20, subrefiector 21 would rotate with respect to reflector 16 when the antenna 15 in a nonvertical position. In FIG. 3, for example, subrefiector 21 would rotate in a clockwise direction with respect to reflector 16 if guy wires 19 and 21 were not present. It is this effect which is detrimental to the successful operation of the antenna and which is prevented through the use of the present invention.

The combination of inflated membrane 18 with guy wires 19 and 20 causes the load transverse to the membrane to be share between the membrane and guy wires. Furthermore, the share of the transverse load assumed by the guy wires is not shared equally by the wires. In particular, guy Wire 19 of FIG. 3 is under greater tension than guy wire 20. This difference in tension in the guy wires causes a counterclockwise moment to be applied to ring 22. An equal and opposite moment is, of course, applied to ring 22 by inflated membrane 18. In accordance with the invention, the stiffness characteristic of the inflated membrane and the guy wires are determined in accordance with conventional engineering techniques so that the moment applied to the subrefiector assembly, and therefore to the membrane, by the guy wires is just sufficient to maintain the desired angular position of the subrefiector relative to reflector 16 as shown in FIG. 3.

The sharing of the transverse load between inflated membrane 18 and guy wires 19 and 20 remains the same for all elevational angles of the antenna. Because of this, the interaction moment between the wire and membrane is just suflicient to maintain the subrefiector assembly in the desired angular position with respect to reflector 16 for changes in elevational angles. The subrefiector assembly therefore maintains its desired angular position for all elevational angles of the antenna.

It should be noted that once the stiflfness characteristics for an embodiment have been determined, the load supported by the structure may be changed in value and as long as the stiffness characteristics are unchanged, the embodiment still operates in accordance with the invention. This occurs because the load is still shared between the inflated membrane and the guy wire in the same proportion.

Although the disclosed embodiment of the invention shows the guy wires located outside of the inflated membrane, embodiments of the invention may be constructed having the .guy wires located within the inflated membrane. Furthermore, embodiments may be constructed using more than two guy wires. Regardless of the location and number of guy wires used in such embodiments, the stiffness characteristics of the wires and the inflated membrane are always selected so that interaction moment between the wires and membrane is just suflicient to maintain the subrefiector assembly in the desired angular position with respect to the parabolic reflector. Embodiments other than that disclosed may therefore be constructed without departing from the spirit and scope of the invention.

What is claimed is: 1. A Cassegrainian antenna comprising a main reflector, a subrefiector, an inflated mmebrane holding said subrefiector in front of said main reflector, and at least two guying members connected between said main reflector and said subrefiector and supporting the portion of the load transverse to said inflated membrane to produce moments on said subrefiector just suflicient to maintain said subrefiector in the same angular position relative to said main reflector for all elevational angles of the antenna. 2. A Cassegrainian antenna comprising a main reflector, a subrefiector, an inflated membrane holding said subrefiector in front of said main reflector, and at least two guying members each of which is connected between said main reflector and said subreflector and lying in a vertical plane passing through the midpoints of said reflectors, said members and said inflated membrane both under tension for all elevational positions of said antenna with said subrefiector in substantial axial alignment with said main reflector when said antenna is in a vertical elevational position, said members and said inflated membrane having stiffness characteristics that for an elevational angle other than vertical result in equal and opposite moments being applied to said subrefiector when said subreflector is in the same angular position with respect to said main reflector as when said antenna is in a vertical elevational position. 3. A Cassegrainian antenna comprising a parabolic reflector, a subrefiector, a truncated cone-shaped membrane, means securing said subrefiector in a substantially airtight manner in the smaller end of said truncated cone-shaped membrane with the reflective surface of said subrefiector enclosed by said membrane, means securing the larger end of said truncated coneshapcd membrane in a substantially airtight manner to the concave surface of said parabolic reflector, means to maintain said membrane in an inflated condition, and at least two guying members connected between said parabolic reflector and said subrefiector, said guying members lying in a vertical plane passing through the midpoints of said reflectors and symmetrically disposed about a line connecting said midpoints, said members and said membrane in its inflated condition both under tension for all elevational positions of said antenna, said members having stiflness characteristics that produce a diflerence in the tensions in said members when said antenna is in an elevational position other than vertical to apply a moment to said subrefiector to maintain said subrefiector in the same angular position relative to said parabolic reflector occurring when said antenna is in a vertical position. 4. A combination for supporting a subrefiector in front of a main reflector in a Cassegrainian antenna comprising an inflated membrane connected between said subreflector and said main reflector and holding said subreflector in front of said main reflector, and

at least two guying members each of which is connected between said main reflector and said subreflector,

said guying members lying in a vertical plane passing through the midpoints of said reflectors and symmetrically displaced relative to a line passing through said midpoints,

said members and said inflated membrane both under tension for all elevational positions of said antenna with said subreflector in substantial axial alignment with said main reflector when said antenna is in a vertical elevational position,

said members having stiffness characteristics that for an elevational angle other than vertical result in a moment being applied to said subreflector to maintain said subreflector in the same angular position with respect to said main reflector as when said antenna is in a vertical elevational position.

5. A combination for supporting a subreflector in front of a main reflector in a Cassegrainian antenna comprising an inflated membrane connected between said subreflector and said main reflector and holding said subreflector in front of said main reflector, and

at least two guying members connected between said main reflector and said subreflector and supporting the portion of the load transverse to said inflated member that produces a moment on said subreflector just suflicient when said antenna is in an elevational position other than vertical to maintain said subreflector in the same angular position relative to said main reflector as when said antenna is in a vertical elevational position.

6. A combination for supporting a subreflector in front of a parabolic reflector in a Cassegrainian antenna comprising a truncated cone-shaped membrane,

means securing said subreflector in a substantially airtight manner in the smaller end of said truncated cone-shaped membrane with the reflective surface of said subreflector enclosed by said membrane,

means securing the larger end of said truncated coneshaped membrane in a substantially airtight manner to the concave surface of said parabolic reflector,

means to maintain said membrane in an inflated condition, and

at least two guying members connected between said parabolic reflector and said subreflector with at least two of said guying members lying in a vertical plane passing through the midpoints of said reflectors and symmetrically disposed about a line connecting said midpoints,

said members lying in said plane having stiffness characteristics that cause these members to support the portion of the load transverse to said truncated coneshaped membrane that produces a moment on said subreflector just sufficient when said antenna is in an elevational position other than vertical to maintain said subreflector in the same angular position relative to said parabolic reflection as when said antenna is in a vertical elevational position.

References Cited by the Examiner UNITED STATES PATENTS 2,888,675 5/1959 Pratt et a1. 343880 2,977,596 3/1961 Justice 343915 X 3,056,131 9/ 1962 McCreary 343--781 References Cited by the Applicant UNITED STATES PATENTS 2,212,128 8/ 1940 Richter. 2,913,726 11/1959 Currie et al. 3,001,196 9/1961 McIlroy et a1. 3,005,987 10/ 1961 Mack et al. 3,098,229 7/ 1963 Raabe. 3,125,758 3/ 1964 Koehler.

HERMAN KARL SAALBACH, Primary Examiner.

M. NUSSBAUM, Assistant Examiner. 

1. A CASSEGRAINIAN ANTENNA COMPRISING A MAIN REFLECTOR, A SUBREFLECTOR, AN INFLATED MEMBRANE HOLDING SAID SUBREFLECTOR IN FRONT OF SAID MAIN REFLECTOR, AND AT LEAST TWO GUYING MEMBERS CONNECTED BETWEEN SAID MAIN REFLECTOR AND SAID SUBREFLECTOR AND SUPPORTING THE PORTION OF THE LOAD TRANSVERSE TO SAID INFLATED MEMBRANE TO PRODUCE MOMENTS ON SAID SUBREFLECTOR JUST SUFFICIENT TO MAINTAIN SAID SUBREFLECTOR IN THE SAME ANGULAR POSITION RELATIVE TO SAID MAIN REFLECTOR FOR ALL ELEVATIONAL ANGLES OF THE ANTENNA. 